Process for crystalline growth employing collimated electrical energy



Dec. 20, 1960 G. w. CLARK ET AL 2,965,456 PROCESS FOR CRYSTALLINE GROWTH EMPLOYING COLLIMATED ELECTRICAL ENERGY Filed D60. 31, 1956 INVENTORS GRADY W. CLARK ROBERT A. LEFEVER ATTORNE PROCESS FOR CRYSTALLINE GROWTH EMPLOY- ING COLLIMATED ELECTRICAL ENERGY Grady W. Clark, Richmond, and Robert A. Lefe ver, Bon Air, Va., assignors to Union Carbide Corporation, a corporation of New York a i Filed Dec. 31, 1956, Ser. No. 631,522 6 Claims. (Cl. 23-301 The present invention relates to a novel process for growing crystalline boules employing collimated electrical energy, and more particularly, to such a process for growing crystalline boules of elemental metals, metal oxides. metal salts, or mixtures thereof.

Various crystalline boule materials have been grown for many years by the well-known Verneuil process (as disclosed in the United States Patent No. 988,230) in which powdered feed constituents are introduced in the boule-growing zone where such constituents are heated and melted by an oxy-hydrogen flame. The molten material drops onto a support rod where it adheres and grows into a boule, and the support rod is gradually lowered away from the oxy-hydrogen flame so that only the growing surface of the boule is maintained in the molten state. One improvement over the Verneuil process involves the use of an auxiliary electric are between an electrode and the boule support to supply additional heat to the combustion gases from the oxy-hydrogen flame.

These known processes employing combustion reactions are limited to the growth of boules having compositions compatible with the chemical reactions occurring in the combustion flame. For example, a process using an oxyhydrogen flame is not satisfactory for growing boules containing elemental metals such as nickel, cobalt and silicon, as the ever-present oxygen reacts with the metal "to form metal oxides which deleteriously contaminate the desired elemental metal in the boule.

Another disadvantage of the original Verneuil process is its limitations in control of heat input to the boulegrowi ng surface. The temperature in the boule-growing zone is determined by the hydrogen-oxygen ratio and since this ratio can only be varied over a relatively narrow range, the temperature control is thus restricted. Furthermore, the heat input is also limited to the maximum temper ature attainable with combustion reactions.

Another disadvantages of the original Verneuil process is its limitation to feed constituents in the powdered solid state.

Principal objects of this invention are: to provide an improved process for growing crystalline boules from feed constituents with complete control of the enveloping gas and thus control over the chemical composition of the boule, to provide a process wherein improved heat control and higher heating temperature are obtained, and to provide'a proces'sutilizing' either gaseous or solid feed constituents for such boule growing. Other objects will become apparent from the ensuing disclosure and appended claims.

In US. Patent 2,858,411, issued October 28, 1958, to- R. M. Gage and entitled Arc Torch and Process, there is described a method and apparatus for collimating an electric arc to form a high pressure electric arc having a higher cross-sectional energy density than arcs theretofore encountered. The term high pressure are, as used herein, is discussed (pages 290 and 326) by Cobine in his book, Gaseous Conductors, published in 1941 by MeGraw-Hill, and is to be understood as relating to self- United States Patent sustaining gas discharges in the general pressure range above atmosphere and generally in the current range of a few to thousands of amperes.

In accordance with one aspect of the present invention, a process for growing crystalline boules from feed constituents is provided in which an electric arc is established between a first electrode and a second electrode positioned therefrom having an orifice passing therein. Concurrently, a gas stream is passed in intimate contact with the first electrode to envelop or contain the arc, and the arccontaining gas stream is constricted through the orifice to collimate the are energy and the gas stream as well as to produce a high-pressure arc and a high thermal content effluent. The latter is passed to a boule-growing zone to heat a boule-growing surface positioned within such zone, and the feed constituents are also passed to the boule-growing zone for heating and production of boulegrowing material therefrom. Finally, a crystalline boule of increasing size is formed by fusing and accumulating the boule-growing material on the boule-growing surface.

The sole source of heat for the boule-growing process of the present invention is electrical energy in the form of an electric arc. This energy is used to heat a gas stream which in turn heats the boule-growing surface. Any gas may be used, providing it will support an arc and does not deleteriously efiect the feed constituents. For example, if a nickel metal boule is desired, an inert gas such as argon may be used to heat the nickel powder feed constituent to produce boule-growing material. The

argon gas stream serves to transfer heat from the electric arc to the nickel powder feed constituent, and does not react with the nickel. On the other hand, if one wishes to grow a sapphire boule from alumina powder, the oxi dation problem does not exist, and oxidizing gases such as oxygen could be used as the heating means although inert gases are equally suitable. Thus, the present invention permits the use of a wide variety of gases as the heat transfer medium and provides a method of growing elemental metal crystalline boules which avoids the problem of oxidizing the feed constituents during such growth. The invention also provides an impoved method of growing boules of metal oxides and metal salts.

As previously discussed, the sole source of heat for the boule-growing process of the present invention is electrical energy which heats a gas stream. Such an arrangement permits the supply of exactly the quantity of heat required for growth of a specified boule by suitable adjustment of the process variables. By regulating the power intensity of the arc and the heating gas flow rate, the temperature of the boule-growing zone may be raised or lowered at will. The present invention provides a simple, yet efficient, method for controlling the temperature of the boule-growing surface and provides a method for maintaining the desired temperature level by supplying heat from a single source, electrical energy. Since an electric arc produces extremely high temperatures, this process has a higher upper temperature limit than the prior art processes limited to combustion reactions.

Another characteristic of this novel process is flexibility in selection of the physical state of the feed constituents. Since an inert gas may be used to heat the feed constituents, such feed constituents may be provided in the gaseous state without fear of reaction with the heating gas. For example, an elemental metal silicon boule may be grown from gaseous silane feed constituent using an inert gas such as argon as the heating medium.

The feed constituent may be introduced directly into the boule-growing zone, may be passed through the orifice with the arc-containing gas stream before entering the boule-growing zone, or may be introduced into the boule-growing zone in any other suitable manner.

The invention will be described in detail hereinafter with reference to the accompanying drawings wherein:

Fig. l is a schematic electrical circuit diagram and vertical midsectional view, parts being in elevation, showing one embodiment of novel apparatus for growing a crystalline boule by the process of the invention, wherein the feed constituents are introduced downstream from the orifice and above the boule-growing zone; and

Fig. 2 is a partial, vertical midsectional view, parts being in elevation, showing an alternate embodiment of novel arc-torch apparatus for growing a crystalline boule by the process of the invention, wherein the feed constituents pass through a hollow stick electrode and the orifice before entering the boule-growing zone.

More specifically, in accordance with the invention, feed constituent powder, for example, nickel metal, is placed in a basket screen 11 within a hopper 12. Powder is periodically sifted out of the screen by intermittently striking an anvil 13 which projects from the top of the screen to the outside of the hopper 12, with a pivoted hammer 14 actuated by a rotating cam 15. A carrier gas stream unreactive with the nickel powder, for example, argon, enters the hopper 12 through conduit 16 and carries the powder through a conduit adapter 17 and conduit 18 into annular feed constituent passage 19 formed between the lower casing 20 and outer concentrically positioned housing 21, the latter two components being leak-tightly connected at their upper ends by a threaded joint. The powdered nickel in the argon carrier gas stream is injected from the feed constituent passage 19 through multiple horizontal passageways 19a into center annulus 26 in the chamber header 22. The powder is then carried by hot efiluent gas to the boule-growing zone 23 in the closed chamber 24. The multiple horizontal passageways 19a are preferably drilled at equal distances around the circumference of the chamber header 22, for example as shown in Fig. 1, six passageways may be spaced at 60-degree intervals.

The heating gas, for example, argon, enters the upper casing 25 through conduit 27 and passes as a stream into the arc nozzle 28 which is axially aligned in the lower casing 20 and comprises one electrode. An electrode holder 29 is axially aligned inside the upper casing 25 and a stick electrode 30, preferably composed of thoriated tungsten or the like, projects from the lower end of the electrode holder 29 into the nozzle 28. The discharge portion of the nozzle 28 tapers to form a constricted outlet or orifice 31 of relatively reduced diameter. The outer wall of the nozzle 28 and the inner wall of the lower casing .20 are spaced to provide an annular coolant passage 32 for the circulation of coolant, e.g., water around the nozzle 28 to prevent overheating. The coolant is fed through conduit 33 to annular passage 32 and then to a coolant reservoir 34 which is axially positioned around the upper part of the nozzle 28 and is leak tightly metal bonded to the lower casing 20. The conduit 33 extends from the coolant reservoir 34 to the annular passage 32 and the water emerging from conduit 33 circulates around the lower part of the nozzle 28 before rising into the coolant reservoir 34 where it cools the upper part of the nozzle 28 and is discharged through conduit 37. The upper end of the coolant reservoir 34 is sealed off against the bottom side of connector 35, and the upper casing 25 is leak tightly attached to the top of connector 35 by, for example, a threaded connection.

The argon gas in the nozzle 28 passes in intimate contact with, and around the stick electrode to envelop or contain the arc. The arc-containing gas is then constricted through the orifice 31 to collimate the are energy and the gas stream as well as to produce a high-pressure arc and high thermal content effluent gas. As previously discussed, the nickel powder is ejected from horizontal passageways 19a into the effluent as the latter passes through center annulus 26 of the chamber header 22,

and is carried into the boule-growing zone 23. This arrangement provides a remarkably high rate of heat generation in the efiluent, which in turn heats the powder feed to produce boule-growing material and also maintains a molten boule-growing surface in zone 23. This material falls through the downwardly flowing hot argon effluent and is fused and accumulated on the upper end of a vertical boule support rod 41 on top of which a seed crystal 42 is placed. As the fusing material accumulates, progressive crystallization is induced on the boule-growing surface to form a boule 43 having the same crystallographic orientation as the seed crystal 42 by gradually lowering the boule support rod 41 vertically downwardly away from the boule-growing zone 23 to maintain the proper temperature gradient through the molten cap of the boule. If a particular crystallographic orientation is not required, the seed crystal 42 is not necessary. Any suitable lowering mechanism can be used, such as vertical pedestal 44 in meshed-gear relationship with horizontally rotating member 45. If desired, the pedestal 44 may be horizontally rotated for greater assurance of uniform boule growth around the vertical axis (horizontally rotating means not shown). It is to be understood that the process variables such as heat input and the boule position may be adjusted in a manner well-known to those skilled in the art. I

If an elemental metal boule .is to be grown, the chamber 24 is preferably sealed from the atmosphere to avoid oxidation. Even if the boule is to be a metal oxide or metal salt, the use of a sealed chamber is desirable as it minimizes dissipation of heat from the boule-growing zone 23, helps to maintain the desired temperature .gradient across the molten boule cap, and avoids misdirection of the falling boule-growing material away from the boule support rod 41 by air currents circulating transversely across the boule-growing zone 23. A sealed chamber also permits operation at pressures above or below atmospheric pressures if these conditions are found desirable.

The stick electrode 30 is connected to the negative side of a direct current source 38 through the electrically conductive electrode holder 29 by lead 39, while the positive side of such source is connected by lead 40 through the electrical housing 21 to the nozzle 28 as the anode.

Instead of injecting the feed constituents into the hot efi luent gas downstream of the orifice 31, as shown in Fig. 1, such constituents may be introduced through conduit 18 to conduit 27 and hence to the upper casing 25 for passage through the entire length of nozzle 28, orifice 31, and finally into the boule-growing zone 23. One advantage of such an arrangement is increased efliciency in heat transfer from the arc-containing gas to the feed constituents. This is due to the intimate contact between the gas and the feed constituents as they pass simultaneously through the orifice 31.

Although intermittent introduction of the feed constituents has been described and illustrated, the process and apparatus of the present invention is adaptable to continuous introduction of the feed constituents. This could be achieved, for example, by introducing such feed through conduit 16.

Furthermore, if the presence of an active gas such as oxygen does not deleteriously effect the particular boule to be grown, such gas could be used in the system pro viding it does not contact the tungsten stick electrode 30 and produce corrosion thereof. This can be avoided, for example, by surrounding the stick electrode 30 with a shield and passing inert gas inside the shield to envelop the stick electrode.

Fig. 2 illustrates a stick electrode, nozzle and orifice assembly in which a hollow stick electrode is used instead of a solid stiek electrode. The feed constituents enter the assembly through the hollow stick electrode 50 which is partially surrounded by the lower casing 51,

thereby providing annularcoolant passage 52 between the lower casing 51 and the outer walls of hollow stick electrode 50. Nozzle 58 is concentrically positioned around the lower casing 51, and the gas stream enters through the annular passage 59 between the nozzle 58 and the outer walls of the lower casing 51. The gas stream and the feed constituents converge at the outlet end of the stick electrode and the arc is enveloped or contained by the gas stream before constriction through orifice 61 in the lower end of the nozzle 58 to collimate the are energy and the gas stream as well as to form a high-pressure arc and a high thermal content effiuent. The efiluent is further processed in the predescribed manner. The lower end of the nozzle 58 is cooled by introducing cold fluid from inlet conduit 62 into the annular space 63, such fluid being discharged through outlet conduit 64.

The apparatus of Figs. 1 and 2 is adaptable to employment of feed constituents in the gaseous state. In such case, the feed constituent gas, preferably diluted by a nonreactive carrier gas such as argon, may be introduced into the hopper 12 through the feed conduit 16, or alternatively may be introduced through conduit 27 into the upper casing 25 and hence to nozzle 28. In either case, the feed constituent gas is heated to its decomposition temperature whereupon elemental metal is formed and deposited upon the boule support rod to facilitate growth of a crystalline boule.

It is evident from the foregoing discussion that the boule-growing material may be either electrically conductive or non-conductive since the electric arc passes between the stick electrode 30 and the walls of the orifice 31, and the boule itself does not form part of the electrical circuit. Thus, the Fig. 1 apparatus would be equally suitable for growth of non-conductive sapphire boules or conductive elemental metal silicon boules.

Although the invention has been described in terms of direct current-straight polarity operation (nozzle positive), direct current-reverse polarity operation (stick electrode positive) is also applicable. Furthermore, alternating current with or without superimposed high-frequency is also practicable.

The unique advantages of the present invention may be further illustrated by the following tests which were successfully performed:

EXAMPLE 1 Growth of nickel boule from nickel powder An arc torch similar to that shown in the embodiment of Fig. 1 was constructed comprising a ;-in. diameter solid tungsten stick cathode positioned within a A-in. diameter water-cooled copper nozzle anode. A powder feed device was positioned coaxially below the nozzle outlet and connected to a feed constituent powder hopper. A carrier gas supply conduit as well as a mechanical tapper to pulse powder down through the feed device were connected to the powder hopper. A ceramic boule support rod was positioned coaxially beneath the torch nozzle as a base for boule growth. To protect the growing boule from oxygen, the entire boule-growing zone beneath the torch was sealed olf from the atmosphere by a 2 /t-in. diameter silica glass tube.

The are torch was operated at 50 amperes and 18 volts (D.C.S.P.), and argon gas was passed down through the electric arc and out of the nozzle at 6 liters per minute. Nickel powder was injected at about 15 pulses per minute into an argon carrier gas stream of 6 liters per minute and then passed into the hot effluent emerging from the orifice. The heated nickel powder dropped onto the ceramic support rod to form a nickel boule.

EXAMPLE 2 Growth of silicon boule from silane gas An arc torch similar to that used in Example 1 (see Fig. l) was used for this experiment. The torch was 6. operated at amperes and 24 volts (D.C.S.P.).. Argon gas at 3.3 liters per minute was passed down through the torch nozzle. Silane gas (7 volume percent in argon carrier gas) was injected into the boule-growing zone directly below the torch nozzle at 3.0 liters per minute (total argon plus silane in stream). Smallglobules of elemental silicon were visibly deposited on the support rod indicating growth of a silicon boule from silane gas by decomposition ofthe gas. Silicon houles have also been grown from silicon powder'using the same apparatus.

EXAMPLE 3 Growth of sapphire boule from alumina powder An arc torch similar to that used in Example 1 (see Fig. 1) was also usedfor this experiment. The torch was operated at ISO amperes and 19 volts (D.C.S.P.). Argon gas at 7.5 liters per minute was passed down through the torch nozzle and argon carrier gas was passed through the power hopper and feed device at 0.75 liters per minute. Alumina powder in the carrier gas stream was fed down the feed tube and out of the feed device into the hot efiiuent at about 18-24 pulses per minute. The heated alumina dropped onto a sapphire support rod to form a sapphire boule.

What is claimed is:

1. A process for growing a crystalline boule from feed constituents comprising the steps of concurrently establishing an electric are between a first electrode and a second electrode positioned therefrom having an orifice therein, passing a gas stream in intimate contact with said first electrode to contain the arc, constricting the arccontaining gas stream through said orifice to collimate the are energy and the gas stream as well as to produce a high-pressure arc and a high thermal content efiiuent, passing the efiiuent to a boule-growing zone and heating a boule-growing surface positioned within such zone by means of such efiiuent, passing said feed constituents to said boule-growing zone and in said high thermal content efiiuent for heating and production of boule-growing material therefrom, and fusing and accumulating such material on said boule-growing surface as a crystalline boule of increasing size.

2. A process for growing a crystalline boule from powder feed constituents comprising the steps of concurrently establishing an electric are between a first electrode and a second electrode positioned therefrom having an orifice therein, passing a gas stream in intimate contact with said first electrode to contain the arc, constricting the arc-containing gas stream through said orifice to collimate the energy by forming a high-pressure arc and a high thermal content eflluent, passing the effiuent to a boule-growing zone and heating a boule-growing surface positioned within such zone by means of the efiiuent, passing said powder feed constituents to said boule-growing zone by means of a carrier gas stream and in said high thermal content efiluent for heating and production of boule-growing material therefrom, and fusing and accumulating such material on said boule-growing surface as a crystalline boule of increasing size.

3. A process for growing an elemental metal crystalline boule from gaseous feed constituents thermally decomposable to an elemental metal comprising the steps of concurrently establishing an electric are between a first electrode and a second electrode positioned therefrom having an orifice therein, passing a gas stream in intimate contact with said first electrode to contain the arc, constricting the arc-containing gas stream through said orifice to collimate the energy 'by forming a highpressure are and a high thermal content efiluent, passing the efiluent to a boule-growing zone and heating a boulegrowing surface positioned within such zone by means of such efiluent, passing a gaseous feed constituent stream to said boule-growing zone and in said high thermal content effluent for heating and thermal decomposition to form an elemental metal therefrom, and fusing and accumulating said elemental metal on said boulegrowing surface as a crystalline boule of increasing size.

4. A process for growing an elemental metal cryst'ah line boule from metal powder feed constituents comprising the steps of concurrently establishing an electric are between a first electrode and a second electrode positioned therefrom having an orifice therein, passing a' gas stream in intimate contact with said first 'electrodeto contain the arc, constricting the arc-containing gas stream. through said orifice to collimate the energy by forming a high-pressure .arc and a .high thermal content eflluent, passing the efiluent to a boule-growing zone and heating a boule-growingsurface positioned within such zone by means of such effluent, passing the metalpowder feed constituents to said boule-growing zone by means of an inert carrier gas stream and in said high thermal content effluent for heating and production of boulegrowing material therefrom, and fusing and accumulating such material on said boule-growing surface as an elemental metal crystalline boule of increasing size.

5. A process for growing a metal oxide crystalline boule from powder feed constituents comprising the steps of concurrently establishing an electric arcbetween a first electrode and a second electrode positioned therefrom having an orifice therein, passing a gas stream in intimate contact with said first electrode'to contain the arc, constricting the arc-containing gas stream through said orifice to collimate the energy by forming a high pressure are and a high thermal content efi1uent,'passing the effiuent to a boule-growing zone and heating a boulegrowing' surface positioned within such zone by means of such efiluent', passing said powder feed constituents to said boule-growing zoneby means of a carrier gas stream and in said high thermal content etfiu'ent for heating and production of boule-growing material therefrom, and

fusing and accumulating such material on said boulegrowing surface as a metal oxide crystalline boule of increasing size.

6. A process for growing a crystalline boule from feed constituents comprising the steps of concurrently estab lishing an electric are between a first electrode and a second electrode positioned therefrom having an orifice therein, passing a gas stream in intimate contact with said first electrode and through said orifice to contain the arc, passing the arc-containing gas stream to a boulegrowing zone and heating a boule-growing surface positioned within such zone, introducing said feed constituents in said boule-growing zone and heating such constituents by passage in said arc-containing gas stream to produce boule-growing material, and fusing and accumulating such material on said boule-growing surface as a crystalline boule of increasing size.

References Cited in the file of this patent UNITED STATES PATENTS 988,230 Verneuil Mar. 28, 1911 1,529,943 Rouvre Mar. 17, 1925 1,587,197 Southgate June 1, 1926 2,011,872 Rava Aug. 20, 1935 2,591,561 Lester Apr. 1, 1952 2,634,554 Barnes Apr. 14, 1953 2,690,062 Burdick et a1. Sept. 28, 1954 2,692,456 Dauncey Oct. 26, 1954 2,697,308 Dauncey et al. Dec. 21, 1954 2,792,287 Moore et a1. May 14, 1957 2,852,890 Drost et a1. Sept. 23, 1958 FOREIGN PATENTS Switzerland Jan. 16, 1933 

1. A PROCESS FOR GROWING A CRYSTALLINE BOULE FROM FEED CONSTITUENTS COMPRISING TE STEPS OF CONCURRENTLY ESTABLISHING AN ELECTRIC ARC BETWEEN A FIRST ELECTRODE AND A SECOND ELECTRODE POSITION THEREFROM HAVING AN ORIFICE THEREIN, PASSING A GAS STREAM IN INTIMATE CONTACT WITH SAID FIRST ELECTRODE TO CONTAIN THE ARC, CONSTRICTING THE ARCCONTAINING GAS STREAM THROUGHH SAID ORIFICE TO COLLIMATE THE ARC ENERGY AND THE GAS STREAM AS WELL AS TO PRODUCE A HIGH-PRESSURE ARC AND A HIGH THERMAL CONTENT EFFLUENT, 